1. Field of the Invention
The present invention relates to microwave resonators, and particularly to a novel structure of microwave resonators which have a signal conductor formed of a compound oxide superconducting thin film.
2. Description of Related Art
Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength in a range of tens of centimeters to a few millimeters can be theoretically said to be merely a part of an electromagnetic wave spectrum, but in many cases, have been considered in the electric engineering field to be a special independent field of the electromagnetic wave, since special and unique methods and devices have been developed for handling these electromagnetic waves.
In 1986, Bednorz and Muller reported (La, Ba).sub.2 CuO.sub.4 showing a superconduction state at a temperature of 30 K. In 1987, Chu reported YBa.sub.2 Cu.sub.3 O.sub.y having a superconduction critical temperature on the order of 90 K., and in 1988, Maeda reported a so-call bismuth (Bi) type compound oxide superconductor material having a superconduction critical temperature exceeding 100 K. These compound oxide superconductor materials can obtain a superconduction condition with cooling using inexpensive liquid nitrogen. As a result, the possibility of actual application of superconduction technology has been discussed and studied.
Phenomenon inherent to the superconduction can be advantageously utilized in various applications, and the microwave component is no exception. In general, a microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss in a recent microstrip line is almost entirely attributable to the resistance of the conductor in a frequency region not greater than 10 GHz, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly enhance the performance of the microstrip line.
As is well known, the microstrip line can be used as a simple signal transmission line. In addition, if a suitable patterning is applied, the microstrip line can be used as microwave components including an inductor, a filter, a resonator, a delay line, etc. Accordingly, improvement of the microstrip line will lead to improvement of characteristics of the microwave component. Therefore, various microwave components having a signal conductor formed of an oxide superconductor have been proposed.
A typical conventional microwave resonator using the oxide superconductor as mentioned above includes a first substrate provided with a superconducting signal conductor formed of an oxide superconducting thin film patterned in a predetermined shape, and a second substrate having a whole surface provided with a superconducting ground conductor also formed of an oxide superconducting thin film. The first and second substrates are stacked on each other within a metal package, which is encapsulated and sealed with a metal cover.
The superconducting signal conductor is composed of a resonating superconducting signal conductor, and a pair of superconducting signal launching conductors, located at opposite sides of the resonating superconducting signal conductor, separated from the resonating superconducting signal conductors. These superconducting signal conductor and the superconducting ground conductor can be formed of an superconducting thin film of, for example, an Y-Ba-Cu-O type compound oxide.
The microwave resonator having the above mentioned construction has a specific resonating frequency .function..sub.o in accordance with the characteristics of the superconducting signal conductor, and can be used for frequency control in a local oscillator of microwave communication instruments, and for other purposes.
However, one problem has been encountered in which the resonating frequency .function..sub.o of the microwave resonator actually manufactured by using the oxide superconductor is not necessarily consistent with a designed value. Namely, in this type microwave resonator, a slight variation in characteristics of the oxide superconducting thin film and a slight error in assembly both cause an inevitable dispersion in the characteristics of the microwave resonator.